Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes formed of porous, electrically conductive sheet material. The MEA includes a catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
In typical fuel cells, the MEA is disposed between two electrically conductive separator plates. The plates may have flow passages formed therein for directing the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, such fluid flow field plates are provided on each of the anode and cathode sides. The fluid flow field plates act as current collectors, provide fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of products, such as water, formed during operation of the cell.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In typical series arrangements, one side of a given separator or fluid flow field plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates. The stack typically includes manifolds and inlet ports for directing a fuel stream and an oxidant stream to the anode and cathode flow field channels. The stack also usually includes a manifold and inlet port for directing a coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the operation of the fuel cell stack. The stack also generally includes exhaust manifolds and outlet ports for expelling unreacted fuel and oxidant streams and reaction products as well as an exhaust manifold and outlet port for coolant water exiting the stack.
Solid polymer fuel cells employing proton exchange membranes and stacks of such cells are compressed in order to improve electrical contact between cell components and to maintain the integrity of compression seals which keep the various fluid streams separate. During the lifetime of a stack, fuel cell thickness can change. For example, MEAs have a tendency to gradually decrease in thickness as a consequence of being subjected to compressive forces. The compression assembly for compressing the fuel cell assemblies preferably applies the desired internal compressive force while accommodating changes in fuel cell thickness. Traditional compression assemblies comprise springs and/or hydraulic pistons, employed either individually or in combination.
When a hydraulic piston is employed in a compression assembly, the fluid pushing on the piston can be an externally supplied fluid or one of the working fluids of the fuel cell stack. Springs are often used as a backup to provide a compressive force if the piston pressure is lost or inadequate for applying the desired internal compressive force for efficient and safe fuel cell operation. In either case, ideally the desired internal compressive force is applied to the fuel cell assemblies over the range of internal pressures attainable in an operational fuel cell stack. Unfortunately, the use of a hydraulic piston adds to the complexity of the fuel cell stack and is a source of unreliability with significant adverse consequences if the piston-based compression system fails. For example, such adverse consequences may include leakage, intermixing and/or contamination of the reactant fluids, chemical reactions between fluids and the fuel cell components, and/or degradation of fuel cell components.
Instead of hydraulic pistons, some conventional fuel cell stacks employ mechanical compression assemblies. For example, it is known to use compressed springs in conjunction with a retention device, such as tie rods or retention bands. The compressed springs expand, in response to reductions in the thickness of stack components, to continue to apply internal compressive force to the fuel cell assemblies.
Optimally, the compression assembly should impart a sufficient internal compressive force to ensure good electrical contact and sealing within the stack without detrimentally deforming the stack components. These requirements define the preferable operating range for the desired internal compressive force. Of course this operating range will vary depending upon the characteristics of the materials from which the stack is made.
The stiffness of the stack components is dependent upon the stiffness of the materials of each component in the stack. For example, the flow field plates may have a high spring rate (that is, a large force produces a small deflection), whereas the electrodes of the MEA typically have a low spring rate (that is, a large force produces a large deflection). The remaining components of the stack are generally stiff and thus have a high spring rate. The net result is that the stack components act as a spring with a varying spring rate, with the MEAs being the main contributor to the spring rate variability. In general, the spring rate of the stack components increases as the internal compression force increases.
Therefore, in a conventional fuel cell stack, the fuel cell stack components (acting like a spring) and the compression springs, act as two spring systems in balance. The substantially rigid stack retention device (for example, tie rods or retention bands) is held in tension and keeps the combined overall length of the stack substantially constant. Accordingly, deflection of the compression springs in either direction is matched by a corresponding deflection of other stack components, such as the fuel cell assemblies. As the compression springs compress, they decrease in length resulting in an increase in the spring force; the stack components simultaneously increase in thickness, resulting in a decrease in the internal compressive force.
In a conventional fuel cell stack, an increase in the internal stack fluid pressure may cause stack components to expand in thickness, resulting in a corresponding reduction in the internal compressive force. The magnitude of the reduction in internal compressive force depends upon the ratio of the stiffness of the stack components and the stiffness of the compression springs.
One approach to limiting reductions in the internal compressive force is to limit the internal stack fluid pressure. This is an unacceptable solution in situations where it is desirable to use higher fluid pressures, for example, to improve fuel cell performance. Another approach to this problem is to increase the spring rate of the compression assembly to resist compression of the compression assembly, thereby reducing the magnitude of stack component expansion. By resisting stack component expansion, reductions in internal compressive force are also resisted. However, a disadvantage of increasing the spring rate of the compression assembly is that a small amount of stack component shrinkage will significantly reduce the remaining compressive force available from the spring. Therefore, in conventional compression assemblies, increasing the spring rate counteracts the primary function of the springs, namely, to deflect over a wide range to apply internal compressive force to the fuel cell assemblies as the fuel cell thickness changes.
In general, a problem with compressed springs is that as a compressed spring expands, its spring force declines, resulting in a deceasing ability to apply compressive force to the stack components. The decline in spring force can be reduced by using a spring having a lower spring rate. A spring with a low spring rate can be pre-compressed so that it will expand and continue to apply an internal compressive force as fuel cell thickness is reduced. Preferably the spring is selected so that the pre-compression force deflects the spring by an amount much greater than the anticipated amount of stack component shrinkage so that the spring will apply the desired internal compressive force throughout the service life of the fuel cell assembly. For example, disc springs (sometimes referred to as Belleville washers) can be made with a spring rate suitable for use in fuel cell compression assemblies.
Thus, in conventional fuel cell stacks the desire to have a low spring rate to accommodate stack component shrinkage, is balanced against the need for an infinite or very high spring rate to counter the effect of changes in internal stack fluid pressure on internal compressive force. In conventional fuel cells, a compromise is typically made between these two conflicting requirements by applying high pre-compression forces to mechanical compression assemblies and limiting stack fluid pressures. Accordingly, there is a need for a compression assembly which can be used over a wide spring deflection range without imposing limitations on the internal stack fluid pressures.
Therefore, the present fuel cell stack incorporates a compression assembly that has a lower spring rate in the stack compression direction and a higher or substantially infinite spring rate in the stack expansion direction.
A restraining mechanism for resisting stack component expansion is included as a part of the compression assembly. The restraining mechanism provides an expansion load path which bypasses the compression mechanism, substantially preventing deflection of the compression mechanism which would otherwise occur when the stack end plates are urged apart, for example, as the stack fluid pressure is increased. For example, in a compression assembly which uses springs to apply a compressive force, when the stack fluid pressures impose compressive forces on the compression assembly, a rigid member is engaged to substantially prevent compression of the springs and movement of the end plates apart. The restraining mechanism is preferably adjustable so that it is effective despite decreases in the distance between the end plates and corresponding deflections in the compression mechanism, such as would occur when there is a reduction in the thickness of the fuel cell assemblies.